HPA said it found a significant statistical association between infection and the consumption of pre-cut spinach, irrespective of retailer. When specific retailers were included in the analysis, the strongest association with infection was found to be with consumption of ready to eat pre-cut mixed salad leaves from a major supermarket chain. Together these findings suggest that one or more types of salad vegetables could have been contaminated.

The Food Standards Agency (FSA), who were part of the outbreak control team led by the HPA, gathered information on the production and distribution of salad vegetables to try to identify the likely source of the outbreak. Investigation of the food chain including practice and procedures throughout each stage of growing, processing, packing and distribution of salad vegetables has not identified a source of contamination. Bagged salad on sale in supermarkets is often sourced from the same suppliers for most leaf types, often with common production lines packing product for several retailers at the same time. This was the situation in this case.

Dr. Stephen Morton, regional director of the HPA’s Yorkshire and the Humber region and head of the multi-agency Outbreak Control Team, said, “This outbreak was fortunately short lived but it was important to see if we could find the source. Our findings suggest that eating mixed leaf bagged salad was the most likely cause of illness. It is however often difficult to identify the source of short lived outbreaks of this type as by the time that the outbreak can be investigated, the affected food and much of the microbiological evidence may no longer be available. As this was an isolated and short lived outbreak there is no specific action for the public to take, but we hope the investigations between the FSA and the food industry will help to prevent further outbreaks of this type from happening again.”